Periodic sound and vibrations of vehicles are usually detectable on smooth road surfaces at speeds typical of highway road systems, i.e., greater than 40 km/hr. These periodic vibrations represent a recurring pattern of vibrations or force variations and may originate in non-uniform conditions of many of the rotating components or elements of the vehicle, such as the engine, driveline, brake rotors, engine accessories and tire-wheel assemblies, as examples.
Periodic vibrations are so termed because, at a fixed forward speed, they are repetitive in nature, recurring with every successive rotation of the causative component(s). It is common for manufacturers to equip test vehicles with various sound and vibration sensors and then observe prominent periodic content in the measurements.
FIGS. 1, 2 and 3 display spectral analyses of such measurements for an example vehicle operated on relatively smooth pavement. The figures show periodic content of rotating tire-wheel assemblies, with the peaks 230-252 in the graphs reflecting vibration peaks at successive orders of rotation. For example, the first peaks 230 (FIG. 1), 242 (FIG. 2) and 248 (FIG. 3) indicate the vibration occurring at the first order of tire rotation, a recurrent pattern occurring at the frequency of the tire rotation. The second peaks 232 (FIG. 1), 244 (FIG. 2) and 250 (FIG. 3) indicate vibration occurring at the second order of rotation, a recurrent pattern occurring at twice the frequency of tire rotation, etc. FIG. 1 plots a spectrum of vibrations of the steering wheel occurring in the fore and aft direction, which is the direction along that of vehicular travel, one of the three orthogonal directions in space defining a coordinate system for measurement. FIG. 2 plots lateral vibrations, which are transverse to the travel direction and parallel to the plane of the travel surface. FIG. 3 plots vertical vibrations, which are transverse to both the lateral and the fore and aft vibrations, i.e., up and down. Accelerations in FIGS. 1-3 are displayed in units of (g), where 1 g is 9.81 m/s.sup.2.
Tire non-uniformities contribute to these plotted vibrations and are caused by structural, geometric and material irregularities of the tire, typically arising due to vagaries of manufacture, resulting in a variety of symptomatic and causal conditions, including, but not limited to, force and geometric variations, axial asymmetry of tread, etc.
Prominent periodic content is readily observed in the figures and contributes to an impression of lack of smoothness expected as the vehicle is operated on the relatively smooth road surface. As can be observed, significant vibrational content is detected at orders including and beyond that of the first. This combined periodic vibrational content contributes to an impression of "lack of smoothness" if high enough in degree.
Often, similar periodic activity is detectable in the measurements of interior sound, where it is common to identify higher order content as responsible for annoyance conveyed through structure-borne acoustic paths. An example of such is shown in FIG. 4, illustrating a spectral plot of in-vehicle sound, measured as sound pressure level (SPL) in mPa, verses frequency. The "A" denotes that the measurements are weighted. A prominent content at the third order of the tire-wheel rotation rate (reference 254) accounts for an impression of an in-vehicle noise characterized by tell-tale bass and relatively long term modulation qualities.
Diagnostic capabilities exist to trace these interior sound and vibrations to their origins, by wheel position. Further decomposition of sources typically incorporates the use of laboratory apparatus for measurements of force variation. There are a number of existing equipment configurations for accomplishing this end, with widespread acceptance and use within the automotive and tire industries.
These laboratory evaluations typically incorporate measurements of the reaction forces of supporting equipment structures as the tire-wheel assembly is restrained and rotated in a manner similar to that occurring on a vehicle on a roadway. Since force systems derive from elastic as well as time-dependent mechanisms, e.g., inertial, it is customary to obtain measurements under a variety of rotational speeds. The faster speeds, then, involve the inertial and the other time dependent mechanisms in addition to the elastic contributions. Still other measurements might involve geometric, mass or physical property variation of the rotating component as it is rotated. These measurements can then be used as indicators of conditions responsible for the road speed excitation of the periodic vehicular sound and vibration.
In the case of force variation of tire-wheel assemblies, periodic force systems are typically assessed on rigidly restrained axle shafts. Assemblies are mounted on rotatable axles which are restrained from translation in all directions. Strategically located force cells are then utilized to measure the reaction forces required to accomplish this fixed translational constraint. In these cases, the tire is constrained to roll against a surface which is either flat (continuous, thin sheetlike metal, driven by suitably arranged rolls providing a flat, rigidly supported surface in the vicinity of tire contact, simulating flat roadway systems) or curved in the case of rigid drive or reaction rolls. Periodic force systems, derived from a single period of the tire rotation, or multiples thereof, can be obtained at any and all speeds of interest.
Some of these force systems are notably speed dependent, such as fore and aft forces as shown in FIGS. 5 and 6. FIG. 5 displays laboratory apparatus measurements of the "second order" of the periodic fore and aft force variation measured in units of Newtons, N, as they depend on speed. FIG. 6 is a similar display showing the "third order" of the periodic fore and aft force variation as a function of speed. Each trace in the two figures represents measurements for a different tire. As can be seen from these figures, the forces are extremely small at reduced speeds. They can, however, achieve sizable amplitudes at speeds typical of roadway usage as illustrated in the figures.
The amplitudes of these forces at highway speeds, furthermore, are indicative of the periodic vehicular excitation. Other modes of tire force excitation, such as radial or lateral force, however, do exhibit measurable levels at low speeds. FIG. 7, for example, illustrates the amplitude of a second order content of radially directed tire-wheel force variation as a function of speed, as measured on a laboratory apparatus of a type available to those skilled in the art. Each trace represents the radial force variation for a different tested tire. FIG. 7 illustrates that radial force variations are detectable at low as well as highway speeds.
The convenience of measurement at low speed enables assessment of tires and implementation of specifications suggestive of maximum permissible roadway usage levels. Many vehicle manufacturers, thus, have set forth various specifications on these force systems (radial and lateral force variations) that are observable at low speeds. Manufacturers of tires for original equipment applications utilize end-of-line (finished product) measurements to accomplish and assure compliance to these specifications on a 100% inspection basis (comparing each manufactured tire against standards of performance). These end-of-line measurements entail force and/or geometric measurements of the tire inflated and mounted on a wheel or split chuck apparatus operated at extremely low speeds, i.e., at tire shaft rotational rates of approximately 1 cps, corresponding to vehicle speeds of less than 10 km/h.